The overall objective of this research is to investigate the molecular mechanisms and genetic control of cellular repair of damaged DNA and to determine the effect of cellular repair processes on DNA replication, on genetic recombination and on the induction of gene mutations. These problems are being studied in a simple eucaryotic organism, the yeast Saccharomyces cerevisiae. Our approach involves characterization of the radiation-sensitive mutants previously isolated, as well as isolation of new repair mutants by different selective procedures. We are studying their effects on related DNA metabolic pathways of mutation and recombination. The effect on EMS-induced reversion of the genes belonging to the RAD52 epistasis group is being determined by measuring reversion frequencies of two EMS-revertible alleles, hisi-7 and ilvi-92 in haploid and diploid RAD ion and rad strains. The four hyper rec mutants we isolated in our collection of mms mutants are being analyzed in great detail for their effect on spontaneous mitotic and meiotic intergenic and intragenic recombination. Repair of mms- and X-ray-induced single-strand breaks is being examined in various mms and rad mutants. Experiments on post-replicational repair following UV irradiation will be carried out in rad ion and rad mutants belonging to the error-prone pathway. A large number of mutants temperature sensitive for growth will be isolated following mutagenic treatment with nitrosoguanidine. These mutants will be screened for whether they are temperature-sensitive for DNA synthesis, RNA synthesis and protein synthesis. Our interest is in mutants that are temperature-sensitive DNA synthesis strains in yeast.